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From: Aurelien Bouteiller (bouteill_at_[hidden])
Date: 2007-07-25 15:51:20

Hi Ralph and everyone,

I just want to make sure the proposed usecases does not break one of the
current open MPI feature I require. For FT purposes, I need to get some
specific hosts (lets say with a better MTBF). Those hosts are not part
of the MPI_COMM_WORLD but are used to deploy FT services (like event
loggers, checkpoint servers, etc). To enable collaboration between
computing nodes and those FT services, I use the usual MPI2 Dynamics
with MPI_Accept/Connect. This means that those different instances of
mpirun needs to share the same orte registry, so that they can establish
the MPI2 connect/accept trough the registered MPI_ports.

This background in place, my first concern is how the deployment maps to
the allocated resources. The nodes used to deploy FT services are
"special". In typical usecase, I get machines with better MTFB, faster
or larger disks by requesting special properties to the resources
allocation manager. I don't want those to be mixed with regular nodes in
the resulting hostfile: these scarce resources should hold only FT
services, no computing processes. As I understand things, I don't see
any way to avoid mpirun to deploy application processes on my "special"
nodes if they are part of the same launch/allocation in your "filtering"
usecase. Currently I proceed to two different mpirun with a single orte
seed holding the registry. This way I get two different hostfiles, one
for computing nodes, one for FT services. I just want to make sure
everybody understood this requirement so that this feature does not
disappear in the brainstorming :]

Next requirement is the ability to add during runtime some nodes to the
initial pool. Because node may fail (but it is the same with comm_spawn
basically) , I might need some (lot of) spare nodes to replace failed
ones. As I do not want to request for twice as many nodes as I need
(after all, things could just go fine, why should I waste that many
computing resources for idle spares ?), I definitely want to be able to
allocate some new nodes to the pool of the already running machines. As
far as I understand, this is impossible to achieve with the usecase2 and
quite difficult in usecase1. In my opinion, having the ability to spawn
on nodes which are not part of the initial hostfile is a key feature
(and not only for FT purposes).

I know there have been some extra discussions on this subject.
Unfortunately it looks like I am not part of the list where it happened.
I hope those concerns have not been already discussed.


Ralph H Castain wrote:
> Yo all
> As you know, I am working on revamping the hostfile functionality to make it
> work better with managed environments (at the moment, the two are
> exclusive). The issue that we need to review is how we want the interaction
> to work, both for the initial launch and for comm_spawn.
> In talking with Jeff, we boiled it down to two options that I have
> flow-charted (see attached):
> Option 1: in this mode, we read any allocated nodes provided by a resource
> manager (e.g., SLURM). These nodes establish a base pool of nodes that can
> be used by both the initial launch and any dynamic comm_spawn requests. The
> hostfile and any -host info is then used to select nodes from within that
> pool for use with the specific launch. The initial launch would use the
> -hostfile or -host command line option to provide that info - comm_spawn
> would use the MPI_Info fields to provide similar info.
> This mode has the advantage of allowing a user to obtain a large allocation,
> and then designate hosts within the pool for use by an initial application,
> and separately designate (via another hostfile or -host spec) another set of
> those hosts from the pool to support a comm_spawn'd child job.
> If no resource managed nodes are found, then the hostfile and -host options
> would provide the list of hosts to be used. Again, comm_spawn'd jobs would
> be able to specify their own hostfile and -host nodes.
> The negative to this option is complexity - in the absence of a managed
> allocation, I either have to deal with hostfile/dash-host allocations in the
> RAS and then again in RMAPS, or I have "allocation-like" functionality
> happening in RMAPS.
> Option 2: in this mode, we read any allocated nodes provided by a resource
> manager, and then filter those using the command line hostfile and -host
> options to establish our base pool. Any spawn commands (both the initial one
> and comm_spawn'd child jobs) would utilize this filtered pool of nodes.
> Thus, comm_spawn is restricted to using hosts from that initial pool.
> We could possibly extend this option by only using the hostfile in our
> initial filter. In other words, let the hostfile downselect the resource
> manager's allocation for the initial launch. Any -host options on the
> command line would only apply to the hosts used to launch the initial
> application. Any comm_spawn would use the hostfile-filtered pool of hosts.
> The advantage here is simplicity. The disadvantage lies in flexibility for
> supporting dynamic operations.
> The major difference between these options really only impacts the initial
> pool of hosts to be used for launches, both the initial one and any
> subsequent comm_spawns. Barring any commentary, I will implement option 1 as
> this provides the maximum flexibility.
> Any thoughts? Other options we should consider?
> Thanks
> Ralph
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